I've been thinking about the seemingly common belief that says if the piston ring gaps are all lined up, an engine will not start/run due to no or too little compression. This myth has to assume that rings do not rotate in their grooves, or else we would have engines randomly die just driving down the road! However, rings do rotate. For evidence, just look at the cylinder walls of any worn engine. You will see the most wear at top (known as "taper") with the resulting ridge that is cut away with a ridge reamer to get the pistons out. In over 50 years of doing engine work, I have never seen an unworn spot where the top ring gap was. Rather, the cylinders always have even wear all the way around. Why? Because the ring is constantly rotating in its groove, wearing the cylinder evenly. Thus, myth busted.
A second point: The diameter of a piston in the ring land area is typically about .035" smaller than skirt diameter at room temperature, or .0175" on each side. Thus, only .0175" of the ring gap is exposed to compression/combustion pressure. Let's assume a generous ring gap of .025". This gives a ring gap area of only .0004375 square inches. Now consider how much empty space there is between the top and second rings, remembering that the piston here is .035" smaller diameter than the cylinder. This space has a cross section area of .006 sq. in., counting both sides of the piston and a total volume of approximately .037 cubic inches. Thus, compression/combustion pressure getting through the top ring gap will quickly and easily find its way to the second ring gap no matter if that gap is halfway around the cylinder or directly under the top ring gap. Again, myth busted!
A second point: The diameter of a piston in the ring land area is typically about .035" smaller than skirt diameter at room temperature, or .0175" on each side. Thus, only .0175" of the ring gap is exposed to compression/combustion pressure. Let's assume a generous ring gap of .025". This gives a ring gap area of only .0004375 square inches. Now consider how much empty space there is between the top and second rings, remembering that the piston here is .035" smaller diameter than the cylinder. This space has a cross section area of .006 sq. in., counting both sides of the piston and a total volume of approximately .037 cubic inches. Thus, compression/combustion pressure getting through the top ring gap will quickly and easily find its way to the second ring gap no matter if that gap is halfway around the cylinder or directly under the top ring gap. Again, myth busted!
